1. Field of the Invention
The present invention relates to a semiconductor integrated circuit (IC), and more particularly to a circuit arrangement having features which permit the prevention of IC malfunctions due to surge and the IC protection against surge.
2. Description of the Related Art
Various supply voltage levels are used with semiconductor integrated circuits (ICs) which are carried aboard vehicles having limited space into which various devices utilizing electrical and mechanical techniques are incorporated. Batteries aboard vehicles are used as sources for IC supply voltages. In incorporating an IC into a vehicle, a user may make a mistake of reversing connections of the IC to power lines connected to the positive and negative terminals of a battery.
FIG. 1 shows an arrangement of a conventional vehicle-IC protection circuit. An IC chip has an input terminal IN, an output terminal OUT connected to a load L1, a power supply terminal VB connected to a node 11 at the positive side of a power supply PW, and a ground terminal GND1 connected to ground potential (car body). A protection diode D10 has its anode connected to ground potential and its cathode connected to the power supply terminal VB.
Even if, in the above arrangement, the positive node 11 and the ground terminal GND1 were wrong connected, the diode D10 would function to protect the internal circuitry of the IC chip against destruction. That is, a surge voltage applied to the chip from the ground terminal GND1 is limited below the forward voltage drop across the diode D10, thereby protecting the internal circuitry of the IC chip.
FIG. 2 shows another vehicle-IC protection circuit arrangement. In place of the diode D10 of FIG. 1, a protection resistor R10 is provided which is connected between the ground terminal GND1 and a common ground line ICL of the IC. If, in this case, the node 11 were wrong connected to the ground terminal GND1, the protection resistor R10 would limit current to the IC chip, protecting the IC internal circuitry.
The IC protection circuit arrangements of FIGS. 1 and 2 have their respective problems. First, in the case of the arrangement of FIG. 1, in order to meet such a mistake of wrong connection of the power supply, the diode D10 is required to have large current capacity. Thus, the diode will increase in size, involving difficulties in building it into an IC chip. As a result, a built-on device has to be used as the protection diode, which will involve difficulties in placing it in suitable position within vehicle with compactness. In the case of FIG. 2, on the other hand, the protection resistor can be built into an IC chip but has a drawback of low withstand voltage for surge. In ICs aboard vehicles, switching operations are frequent and thus surge voltage is liable to occur all the time due to counter-electromotive force by inductance. Therefore, the surge voltage between the power supply and ground may destroy the protection resistor. For this reason, the arrangement of FIG. 2 provides decreased reliability for vehicle ICs.